allison performance ???

[mad6mac2000]

Hello all I am looking for some help with a allison 643 transmision, I
am very peticular and it is starting to bother me a bit. We are getting
some weird shifts on our bird right now, 1st to second is fine and
firm, 3rd to 4th is some what troubling on the climb through the rpm
range, I get a rpm change 2 times before shifting into 4th gear. It is
almost acting like it has 5 gears, is this the torque converter locking
up in the 3rd gear range? then shifting to 4th? I guess I think it
should just shift 1..2...3....4. I have checked the shift points
compared to rpm to speed and they are right on.

this trans is the allison MT643 not 640

Mac and Carrie Good
1978 31' FC NA